A variety of data communications systems exist for coupling data between end users. Such systems utilize a number of different topologies and protocols. One commonly used arrangement is the Ethernet system in which end users are all connected to one another via a coaxial cable. Deployment of this system has waned due to the need to install coaxial cable. On the other hand, systems which utilize twisted pair wires, such as the 10BaseT system, have experienced widespread growth. In any of the foregoing systems, there are two communications modes. In the busy mode, data is coupled between the end users and, in the idle mode, end-user data is not coupled.
Most recently, a higher-speed version of the 10BaseT system, known as 100 Mbps Fast Ethernet, is being proposed which increases the nominal data rate tenfold. To lessen implementation costs, this proposed system is compatible with the half-duplex Ethernet and 10BaseT protocol. However, the higher speed in this proposed system necessitates the use of distortion compensation devices, such as automatic gain control (AGC) circuits, equalizers and NEXT (near-end crosstalk) cancellers.
Equalizers and NEXT cancellers have long been used for distortion compensation and implemented in either of two classes. In the first, the amount of distortion compensation provided is fixed, while in the second, the distortion compensation provided varies to match the time-varying distortion introduced in the communications channel. Due to the varying nature of the distortion compensation provided, this second class of devices is referred to as being "adaptive".
Adaptive NEXT cancellers and equalizers adjust the amount of distortion compensation required. This adjustment or adaptation process is commonly referred to as convergence and requires a finite time interval. During this time interval, the amount of compensation provided is not optimum and, as a result, there is a greater likelihood of data errors. While data communications could be inhibited during such adaptation intervals, this procedure is inefficient and must be repeated during each idle-to-busy mode transition.
It would therefore be desirable if a technique could be developed for communications systems having busy and idle modes which eliminates the need to converge adaptive distortion compensation devices to their optimum settings after each idle mode.